An ever-increasing demand, primarily by consumers, for higher data rates and higher quality wireless communication systems results in new and continuously developing standards. This compels wireless network operators and service providers to install new or upgraded infrastructures to accommodate these better service standards. Hardware in general is a more expensive component of the wireless communications infrastructure as compared to software with a result that hardware upgrades are more expensive compared to software upgrades. One-way to mitigate this is to have hardware that can work efficiently over a large frequency bandwidth and therefore accommodate the ever-increasing data rate and quality standards and thereby less frequent upgrades.
From a consumers perspective mobile terminals or user equipment should also be able to operate in several frequency bands so that they can work with different standards. Furthermore, having mobile terminals with wideband hardware makes them usable in more operator networks and more countries around the world. Having wideband hardware also provides the capability of using the same terminal for different applications in a system.
A common component in wireless communications systems that may benefit from having a wide operating bandwidth is the radio frequency (RF) power amplifier, which delivers (in a wireless transmitter) a high frequency signal with a required RF power to the antenna.
Furthermore, since the energy for the RF power amplifier to drive a load is generated by a power supply, the average efficiency (defined as the ratio of the average output power to the average supply DC power) is to be considered in power amplifier design. In base stations, high efficiency power amplifiers translate into lower energy cost for operating the power amplifier as well as lower energy consumption by cooling systems, which are typically used with high power amplifiers in base stations. In the case of mobile terminals, a high efficiency power amplifier results in longer battery life.
To achieve higher data rates, standards implement complex modulation and multiplexing schemes such as quadrature amplitude modulation (QAM), orthogonal frequency division multiplexing (OFDM), and other multi-carrier schemes. These signals present high spectrum efficiency, but they also have high peak-to-average power ratio (PAPR). This means that the power amplifier is required to manage signals with a large time varying envelope. Such high values of PAPR implies that the amplifiers operates mostly at an average output power that is much lower (back-off condition) than its attainable saturated output power, which reduces the overall efficiency.
Designing high efficiency power amplifiers for high PAPR signals in a wide frequency band is challenging. The active device, typically a transistor is subject to various electrical constraints on its performance. Different topologies of active devices have been implemented to mitigate constraints on the individual active devices. Well-known power amplification topologies that provide high efficiency for high PAPR signals are for example, load-modulated amplifiers (such as the Doherty Power Amplifier), outphasing amplifiers and push pull amplifiers. Amplifiers are classified according to their circuit configurations and mode of operation and are designated by different classes of operation such as class “A”, class “B”, class “C”, class “AB”, etc. These different amplifier classes range from a near linear output but with low efficiency to a non-linear output but with a high efficiency. Each of these classes has a different bias point or position of the Q point for operating the amplifier. In order for the active devices to operate efficiently in a particular topology, appropriate bias conditions must be defined for the device. Transistors are usually biased using a constant voltage source.
The Doherty amplifier is an example of multi-branch amplifier topology composed of a main active device (commonly denoted as carrier amplifier) operating in class-AB providing signal amplification for all input signal levels, at least one auxiliary active device (commonly denoted as peaking amplifier) operating in class-C providing signal amplification starting from a predefined signal level, an input analog power divider for splitting the input signal between the carrier amplifier and the peaking amplifier, a non-isolated Doherty output power combiner for combining the outputs of the carrier amplifier and the peaking amplifier which includes quarter wavelength transformers, and 50 Ohms lines inserted at the input of the peaking amplifiers and/or carrier amplifier to balance the delay between the branches of the Doherty amplifier. However, Doherty amplifiers have limited operational bandwidth and are required to have quasi-open output impedance at the output of the peaking branch, which limits its operational bandwidth. The Doherty amplifier also needs impedance inverters, which cannot be implemented in very large bandwidth.
The outphasing amplifier topology splits an input signal into two constant envelope phase modulated signals that are amplified by high efficiency non-linear amplifiers and then combined at the output. However the outphasing amplifier is also limited in bandwidth due to the narrow-band power combiner used in its structure.
The push-pull amplifier is another amplifier topology that has high peak energy efficiency. Push-pull amplifiers, utilize two transistors that are biased in class B (near the cut-off region). Each transistor works for half of the input signal cycle and delivers current to the load in the corresponding half cycle. To ensure proper on/off cycles, transistors of different types are needed in push-pull amplifier. For example using bipolar technology, one of the transistors has to be of NPN type and the other transistor should be of PNP type. For FET transistors, one of the transistors has to be of N-channel type and the other transistor should be of P-channel type. In some applications two types of transistors cannot be used. If the same types of transistors are used in push-pull amplifier, one transformer is needed at the input of the amplifier and one transformer is needed at the output of the amplifier. Using transformers limit the operating frequency band of the amplifier and results in larger circuit size.